The present invention relates, in general, to apparatus for producing uniform strands of glass fiber from different sections of a fiber glass bushing, and, more particularly, to a remotely controllable three-terminal circuit for regulating the current flow in, and the temperature of, sections of a fiber glass bushing.
Glass fibers are produced by drawing multiple streams of molten glass at a given rate of speed through orifices or nozzles located in a heated container, known in the fiber glass industry as a bushing. The bushing contains molten glass which is electrically heated and maintained at a given temperature to provide molten glass at a desired viscosity at each of the orifices. The maintenance of a uniform temperature across the face of the bushing; that is, across the area of the bushing on which the orifices are located, is important in providing uniform fiber formation at each orifice.
The fibers drawn from the orifices of a bushing are gathered, after they solidify, into one or more strands, which are then collected on a collet to produce one or more forming packages. In recent years, bushings have increased in size so that bushing having 800 to 2,000 or more orifices are commonplace. It is also common practice to produce more than one strand from larger bushings, and to wind those strands on a single collet to produce corresponding forming packages. Typically, this is accomplished by using one side of a bushing to produce one strand, and the other side to produce a similar second strand. Splitting the bushing in this manner, to produce more than one strand, requires precise control of the bushing temperature from side to side, so that the strands so produced and collected on a collet will have the same yardage, that is, the same number of yards per pound of glass in a formed package, collected over a given period of time. Even small variations in temperature between the two sides of a split bushing will produce substantial differences in the fibers produced at the orifices, and this will be evidenced by a substantial variation in the weights of the forming packages produced by the respective strands.
In recognition of the foregoing problem, various attempts have been made to compensate for such differences, for example, by adjusting air flow to the bushing, adjusting fin cooler placement under the bushing orifices, adjusting terminal clamp positions on the bushing, and the like. Adjustments of the bushing structure are, at best, a function of operator skill, and fall short of being satisfactory, since such changes are manual, time consuming, and imprecise. An improvement over such mechanical adjustments is the system shown in U.S. Pat. No. 4,024,336, which describes a system which controls two sides of a bushing by using two full-wave variable impedance devices to regulate current transmitted to the bushing by a power transformer. Although this device permits the relative yardage of two strands produced from a split bushing to be manipulated, the device has not always been effective because, as a change on one side of the bushing occurs, it is always accompanied by a change on the other side. Further, the system described in this patent requires the use of a temperature controller on each side of the bushing, as well as two full-wave variable impedance devices, and, thus, is a costly system on a per-bushing basis.
In U.S. Pat. No. 4,546,458 and assigned to assignee of the present application, the disclosure of which is hereby incorporated herein, a bushing controller is described in which a variable impedance shunt is connected in parallel with the bushing and with the secondary of the power transformer which feeds current to the bushing. The variable impedance is provided by a potentiometer consisting of a resistance element having a wiper arm, or tap. The resistance element is connected across the ends of the bushing, and the tap is connected to a center point on the bushing, so that movement of the tap alters the impedance connected across the two halves of the bushing.
While the system described in U.S. Pat. No. 4,546,485, effectively controls the current in the bushing from side to side, that system has been found to be unreliable, since it requires periodic mechanical adjustment of a moveable tap which is normally located close to the bushing, and is subject to contamination by the moist environment and the chemical binders normally present in a glass fiber-forming area. Further, the mechanical adjustment of a potentiometer tap lacks precision, and, thus, does not provide totally satisfactory control.
Accordingly, a need exists for a system which will produce a precise and reliable control of the temperature of the forming surface and of the orifices in a fiber glass bushing to enable such a bushing to produce consistent fibers throughout the bushing. Further, a need exists to provide a control system which remains free of contaminants for long-term reliability, and which can be controlled from an area remote from the fiber-forming area, so that numerous bushings can be regulated from a central location.